On Thu 10 Nov 2005 07:58:19p, Thus Spake Zarathustra, or was it Richard?

I don't think they'd frighten the fish.  However, make sure that you have a
very secure full cover on the tank.  Cats like to go "fishing".

I have cats and aquariums together for about 40 years.  Never had a
cat catch a fish or a fish terrorized.  Even had one cat that would
straddle the lamp with all 4 feet in the water.  Fish would come up to
look at him.
Currently have a huge pleco in a 55. My new cat will rush up to watch
when it moves but that is all.
Did have a cat injure a turtle, however.

When i was a kid i had a hamster, and the family
cat would jump on top of the shelve the hamster
aquarium was and just sit in from of the hamster
cage.  Just sat there chilled out.

I'd say make sure the cover can withstand a clumsy cat lying and prancing
all over it.

My parents aquarium (containing an Oscar... sorta like a big pirahana) had
their cat fall in and break stuff a couple of times.  Just because it liked
to hang out and watch the fish.  I'd recommend one of those light/cover
jobbies that is fairly sturdy.

Make sure the aquarium is securely covered.   I had a cat years ago
who stood on top of the aquarium desperately trying to get inside and
at the fish.  She never figured out the door to get inside the tank and
at the fish was the door SHE WAS STANDING ON.

As far as whether it frightens the fish to have this cat looking
at them thinking "dinner is served" I don't know.  I wondered the
same thing.

I've had aquariums and cats for years.  No problem.  However, none of my
cats ever watched the fish.